Sea sleds of the type with which the present invention is most nearly associated are primarily used for entrenching submerged elongated structures, such as pipelines and the like.
Various systems for laying pipelines along the sea bottom have been proposed and utilized in the past (see, for example, U.S. Pat. No. 3,751,927). Certain of these systems provide a sea sled having jet nozzles directed to fluidize the sea bottom and form a trench for receiving the pipeline. Air jet type eductor systems are provided for removing the cuttings or slurry formed by the jet nozzles from the trench. An improved system which operates efficiently at water depths in excess of 150-200 feet is described in said U.S. Pat. No. 3,877,238 and the disclosure thereof is incorporated herein by reference.
In underwater towing, normally the only force measured is the force acting on the tow line connecting the underwater tow to the surface towing vessel. Frequently, the tow or sea sled, will have an attitude other than horizontal or in alignment with the pipeline; that is, the sled may be tilted about one or more of its longitudinal, lateral and vertical axes. This slewing of the sled may be caused by uneveness in the ocean bottom or obstructions in the path of the sled or due to action of the tow line on the sled. Thus, the sled may be towed in a direction other than the desired one. Additionally, the sled may be subjected to horizontal jerking of the tow line which results in lifting the sled off the ocean bottom. Further, the tow line may have insufficient weight to match the length of tow line available.
Prior to the development of this invention, there was no accurate means available for remotely indicating the position of the sea sled relative to the ocean bottom or for measuring the forces actually imposed on the sled by the tow line and movement of the towing vessel located on the surface (hereafter for convenience, but not by way of limitation, referred to as the "tow barge" or merely "barge"). The operator therefore had no precise way of knowing whether the sled was being pulled too far to the right or left or whether it had hit an obstruction. Heretofore, the operator could rely only on highly inaccurate systems, such as a three-pulley in-line arrangement that measured the deflection of and tension on the towing cable at the barge. One such system is the so-called "Dynaline" manufactured by Martin Decker. A major disadvantage of this type of system is that it provides an indication only of the forces acting on the tow line at the barge on the surface and gives no indication of the attitude of or forces acting directly on the sled at the sea bottom.
Another means that has been used in the past to monitor the position of the sea sled consists of load cells mounted on the sled in such a way as to indicate contact of the pipeline with rollers mounted on the sled and straddling the pipeline. Examples of such load cells and remote indicators associated therewith are shown, for example, in U.S. Pat. No. 3,507,126. Devices of this type provide a measure of the lateral off-course movement of the sled to permit the barge operator to adjust the sled position by moving the tow barge in the opposite lateral direction to avoid damaging the pipeline. Such indicating means, however, do not allow for an accurate determination of the attitude of the sled relative to the ocean bottom (and thus the pipeline resting on the bottom) nor of the forces acting on the tow line at the sled.
Other barge mounted measuring devices have been employed in the past with no greater success than the systems mentioned above. Because of the incomplete information and inaccuracies inherent in such prior art systems, the barge operator encountered substantial difficulties in accurately controlling the sled relative to the pipeline as it moved over the ocean floor. As a consequence, the operator could not efficiently maintain the sled in proper operational relationship with the pipeline. Also, he could not readily adjust the sled attitude with any degree of certainty to overcome obstacles encountered by the sled. Nor could he conveniently and accurately reposition the sled relative to the pipeline. Further, overcompensation for incorrect towing sometimes resulted in over-turning the sled, which could damage it or rupture water or air supply lines connected to the sled. Costly down time delays would then be encountered.
The present invention was developed to provide a system for accurately monitoring the forces acting on the tow line in such a way as to provide a precise indication of the position of the sled with respect to the ocean bottom. As will be described more fully hereinafter, the invention utilizes the vector relationships among forces imparted to the tow line by the sled. These forces are measured at the sled and are displayed on a console located at the operator's station on the barge.
The system of this invention permits an operator located on the towing vessel to accurately continuously monitor the attitude and position of the sea sled as well as the various forces acting on the tow line. The operator can then make necessary corrections to the tow to compensate for attitude and/or direction changes of the sea sled in towing and to properly reposition the sled in relation to the pipeline. This is accomplished by changing the location of the tow barge in the usual manner, e.g., by shifting the barge to the right or left or moving it forward or backward, as necessary.
In order to fully appreciate the significance of this invention, it will be helpful to understand the basic mechanics of towing a subsurface structure. The motion of the sea sled on the ocean floor is determined primarily by the pull of the tow line exerted from the towing barge.
Due to ocean currents, the sled will normally not be directly behind the tow barge, but will be laterally offset to an extent depending on the strength and direction of the prevailing ocean currents. The sled movement is effected and controlled by movement of the barge, but movement of the sled may not directly correspond to movement of the barge. Thus, one concern of the operator is the actual effect on the sled of the pulling force exerted by the tow barge. Stated another way, using the present invention, the operator is concerned with the net vectors of forces acting on the sled which will tell the operator in what direction and under what force the sled is being pulled. Knowing these, the barge operator can move the barge on the surface to obtain an optimum tow of the sled on the sea bottom.
It is also helpful for the operator to be able, in effect, to "see" the sled to avoid towing it into a potentially detrimental situation. If we assume a sled weight of 50 tons (sled weights typically range from 40 to 80 tons or more), if a vertical force of 25 tons were applied to one side of the sled, that side would lift off the ocean bottom. If the operator were to continue to move the barge in such a manner that a vertical force of slightly more than 25 tons were continued to be applied to the lifted side of the sled, an overturn condition would occur. The present invention provides a system for indicating the vertical and other forces on each side of the sled and the sled angle with respect to the horizontal reference surface so that the operator can move the barge in such a way as to reduce any excessive force acting on one side of the sled as required to avoid the potential overturn condition. The actual adjustments are made by the operator controlling sets of winches connected to anchor chains on the barge. By pulling in or paying out line to the barge anchors, the operator can move the barge on the surface which in turn will, through the sled tow cable, cause the sled to move appropriately. This type of barge movement system is well known and forms no part of this invention per se.
This invention is particularly useful where two systems are connected to the same sled, each system having different strengths. Such would be the case where water or air supply hoses are spooled off the stern of the tow vessel and the sled tow line is spooled off the bow. Since the bow and stern of the tow vessel move in different modes, it is mandatory that each line be given its optimum configuration but that the hose lines be given preference. This is because the hose lines are most sensitive to overload situations, they are more difficult to replace and they are considerably more expensive than the tow lines. It is therefore desirable that the hose tensile force be kept below its maximum permitted value. Failure to do so risks hose rupture due to differential movement between the sled and the stern of the towing vessel.
It is an object of this invention to provide a system for continuously monitoring the attitude and directon of movement of an underwater tow.
More particularly, the system of this invention has for its objects the following:
To determine whether the length of tow line for the weight of chain used is too short or too long.
To determine whether the tow line is pulling the sled straight ahead or to the right or to the left.
To determine the magnitude of corrective action necessary to force the sled to move right or left.
To determine whether the sled has encountered an obstruction which tends to force the sled off its intended course horizontally or vertically.
To determine the effect of the tow line system on the sled to give an evaluation of the sled's ability to perform its function as the tow vessel responds to the ocean environment.
To indicate whether the sled is falling into a trench behind trenching mechanisms when used in conjunction with such mechanism: i.e., the sled pitches up at the forward end.
To indicate whether the sled encounters softer material: i.e., the sled pitches down at the forward end.
To determine the optimum required weight of tow chain.
To determine the condition for overturn of the sled.
To provide the optimum safety to the weakest of two systems when two tow systems are connected to the same underwater tow.